1. Field of the Invention
The present invention relates to a layout in a display panel in which an electroluminescence element (hereinafter simply referred to as “EL element”) is used as a display element in each of pixels arranged in a matrix.
2. Description of the Related Art
Display devices which use EL elements, in particular, organic EL elements which are current-driven light emitting elements as a display element in each pixel, are known. Among EL display devices, active matrix display devices in which a transistor (thin film transistor or “TFT”) is provided in each pixel for individually driving, for each pixel, the organic EL element provided in each pixel are now a focus of development.
FIG. 1 exemplifies an equivalent circuit corresponding to a pixel in an active matrix display device. A gate line GL is provided along a horizontal scan direction (row direction) of the display device and a data line DL and a power supply line PL are provided along a vertical scan direction (column direction) of the display device. Each pixel comprises a selection transistor Ts which is an n-channel thin film transistor, a storage capacitor Cs, a p-channel element driving transistor Td, and an organic EL element 55. The selection transistor Ts has a drain connected to a common data line DL which supplies a data voltage to pixels positioned along the vertical scan direction, a gate connected to a gate line GL for selecting pixels positioned along the horizontal scan direction, and a source connected to a gate of the element driving transistor Td.
The element driving transistor Td is a p-channel thin film transistor and has a source connected to the power supply line PL and a drain connected to an anode of the organic EL element 55. A cathode of the organic EL element 55 is formed common to the pixels and is connected to a cathode power supply CV. One electrode of the storage capacitor Cs is connected between the gate of the element driving transistor Td and the source of the selection transistor Ts. The other electrode of the storage capacitor Cs is connected to a power supply of a constant voltage such as, for example, ground and a power supply line.
In this circuit, when the gate line GL is set to the H level, the selection transistor Ts is switched on, a data voltage on the data line DL is supplied via the selection transistor Ts to the gate of the element driving transistor Td, the element driving transistor Td allows a drive current corresponding to the gate voltage of the element driving transistor Td to flow from the power supply line PL to the organic EL element 55, and light is emitted from the organic EL element 55 at brightness corresponding to the drive current. The data voltage on the data line DL is supplied to the storage capacitor Cs in addition to the element driving transistor Td and a voltage corresponding to the data voltage is stored in the storage capacitor Cs. Therefore, even when the gate line GL is set to an L level, the element driving transistor Td continues to supply the drive current because a voltage stored in the storage capacitor Cs continues to be applied to the element driving transistor Td, and, thus, the organic EL element 55 continues to emit light at brightness corresponding to the drive current.
FIG. 2 is a conceptual view schematically showing a planar structure of a panel 100 used in an active matrix organic EL display device having a built-in driver as disclosed in Japanese Patent Laid-Open Publication No. 2001-102169 (hereinafter referred to as “Reference 1”). In FIG. 2, the outermost solid line represents a transparent panel substrate 102 and a display region 104, shown by a dotted line and in which the above-described pixels are arranged in a matrix form, is positioned at a position slightly above the center of the panel substrate 102. A horizontal scan driver circuit 106 (hereinafter referred to as “H-related driver”) which is connected to the data line DL is formed along an upper side of the display region 104 and vertical scan driver circuits 108 (hereinafter referred to as “V-related driver”) which are connected to the gate lines GL are formed along the right and left sides of the display region 104. These drivers 106 and 108 comprise a thin film transistor or the like which is formed simultaneously with the thin film transistors provided in each pixel.
The thick solid line extending in the display region 104 along the vertical direction indicates the power supply line PL. Individual power supply line PL is connected to a wide portion 110 in the horizontal direction which extends along the lower side of the display region 104 and forms a comb shape as a whole. The wide portion 110 is further connected, near the center of the wide portion 110, to another wide portion 112 extending along the vertical direction. The wide portion 112 is connected to an input terminal T1 for the drive power supply placed at the lower side of the organic EL display panel 100.
In addition to the terminal T1, a plurality of additional terminals, including a cathode terminal T2, a terminal T3 connected to the V-related driver 108, and a terminal T4 connected to the H-related driver 106, are located on the lower side of the organic EL display panel 100.
A line for supplying power to the H-related driver 106 extends from the terminal T4 at the lower side of the display panel 100 and along the lower side, right side, and upper side of the display region 104 and reaches a position which is almost at the left end of the upper side. A line for supplying power to the V-related driver 108 located on the left side extends from the terminal T3 along the lower side and the left side, and reaches the upper end of the left side. A line for supplying power to the V-related driver 108 placed on the right side extends from the terminal T3 along the lower side and the right side, and reaches the upper end of the right side. In actual devices, a shift register or the like comprising a TFT is provided in the rectangular region shown in the figure as the H-related driver and the V-related driver. A power supply line for driving these circuit elements extends to an end portion of the driver.
Japanese Patent Laid-Open Publication No. 2004-4797 (hereinafter referred to as “Reference 2”) discloses a display device in which the external connection terminal is provided on the lower side, the H-related driver is provided on the upper side, and the V-related driver is provided on the left side. As shown in FIG. 7 of Reference 2, a line for a higher voltage power supply (Vdd) of the power supply lines of the driver extends from the lower side to the right side and supplies a higher voltage power supply to the H-related driver on the upper side and further extends from the H-related driver to the V-related driver formed on the right side. A line for a lower voltage power supply (Vss) extends from the lower side to the left side, supplies the power supply of the lower voltage to the V-related driver, and further extends from the V-related driver to the H-related driver on the upper side. In this manner, the two power supply liens are placed in rotations opposite to each other and only one of the lines is placed on the side on which neither the H-related driver nor the V-related driver is provided. With this configuration, the space required for the line can be reduced.
In the device of Reference 1, the lines for supplying power to the H-related driver and to the V-related driver are provided on four sides of the display device. At the lower side, the line for the driver and the wide portion 110 connected to the power supply line PL are provided, and a contact region extending from the cathode terminal to the common cathode electrode of the EL elements is also provided. Because of this structure, it is difficult to reduce a width of a portion between the display region 104 and the panel substrate 102, that is, the frame portion, on the side on which the external connection terminal is formed. In addition, because the width of the frame portion of the lower side is larger compared to those of the other sides, there is a constraint in the layout when such a display panel is integrated into various devices as a display device.
Other devices are known having the V-related driver 108 placed on one side at the left or right sides, rather than having two V-related drivers 108 on the both sides. However, in these devices also, a line for supplying power must extend into a portion of the lower side, limiting reduction of the width of the frame portion.
The space required for the lines can be partially reduced by employing the structure of the device of Reference 2 in which one of the two power supply lines is laid out in a clockwise manner and the other power supply line is laid out in a counterclockwise manner, but in such a case the line itself is elongated. In particular, in the structure of Reference 2, because a high-speed operation is required for the H-related driver, the power supply line Vdd for supplying the high voltage power supply extends from the external connection terminal to the H-related driver and further to the V-related driver, as described above. In other words, the power supply line Vdd extends from the lower side of the panel and along the right side, upper side, and left side in this order. Thus, the total cumulative length of the power supply line Vdd which extends along the four sides of the panel is great, even in comparison with the power supply line Vss for the lower voltage. When the line is elongated in this manner, additional factors such as, for example, generation of voltage drop and increase of power consumption must then be considered.